... the king of all birds." December 26 is St. Stephen's Day,
the day that Irish boys traditionally hunted the wren and carried
it from house to house, and the day that mummers still parade in
their oatstraw suits in a few localities. The symbology of all
this is less than limpid.

One of the early words for "wren" is "drešn". (The umlaut, by
the way, is a recent scholarly addition to indicate that the word
has two syllables, not a feature of traditional OI spelling.)
One of the "etymologies" of "drešn" offered in Sanas Cormaic,
s.v. "dris" #475, is intriguing:

"dreaan... no drui-en .i. en doni faitsine"

= wren... or druid-bird, i.e. a bird that makes prophecy

LEIA deems this to be a popular etymology, but one which was
widely accepted. "Drešn", whose second syllable is the mark
of the diminutive, is clearly a cognate of Welsh "dryw", a
word which can also mean "seer". The identification of the
little bird with magic is thus not limited to Ireland.

What does the GPC dictionary have for an etymology of the
Welsh name for the bird?